Over a period of many years, a number of methods have been suggested whereby chromium compounds could be recovered and reformulated for tanning purposes. Each of these procedures, however, involved a great many individual steps supplemented by constant chemical analyses in order to insure uniform composition in the chemical tanning solutions. In fact, so complicated did these procedures become that their costs were excessive when compared with the use of fresh chemicals. This led to their very brief use during war times or other periods when the shortage of chromium compounds became critical.
At the present time, with no shortage of chromium, there has arisen a new obstacle to the discarding of spent or excess chromium liquors. This is the objection to certain heavy metals in industrial effluents because of the untoward effect they exert upon subsequent waste treatment processes.
The loss of soluble chromium compounds in tannery effluents has gradually increased over a period of years because it was cheaper to buy fresh supplies of chrome salts than to incur the expense of chrome recovery. Another factor contributing to this discharge of chromium salts was the desire to speed up the tanning process by forcing penetration of chromium salts by using higher concentrations of chromium salts. This results in an increased tanning rate with commensurate economics in equipment cycling and labor per unit of production. At the same time, however, it increases the amount of chromium discharged in the tannery effluent as well as the percentage of chromium lost from the leather making process.
From a number of surveys made for various tanneries, it has been found that the most efficient tanneries use about two-thirds of the total chromium purchased and discard one-third in their plant effluent. Less efficiently managed tanneries may discard up to one-half of the chromium purchased.
All tannery waste generated by washing, soaking, dehairing, bating, pickling, chrome tanning, dyeing, fat-liquoring, and finishing combines to form an effluent that is predominantly alkaline and contains the waste chromium in its precipitated hydroxide or metal chromite form. This can be settled by conventional catch basin systems resulting in the production of sludge containing not only chromium compounds but grit, manure, grease, and other suspended or settleable solids. However, the collection of sludge for disposal entails a great deal of expense.
If the chromium is diverted from the total plant effluent, then the volume of sludge to be disposed of is significantly reduced.
Over a period of many years, it has been conventional to manufacture leather by processing hides and skins through a series of steps some of which make use of different machinery and equipment (e.g. paddle vats and drums). Many tanneries are very old and sewer lines have not been laid to segregate the various waste streams. In addition, the hides in process may be moved over a considerable area and the chemical wastes may be widely scattered by draining. As dumped from tanning drums, the chrome tanned "blue stock" lands in a pile with the spent chrome liquors following the floor slopes to the drains and sewers. When the blue stock is piled for draining, it is frequently moved into other plant areas and excess chrome finds its way into other drains. Therefore, it is obvious that segregation of chrome liquors is almost impossible in many plants.
More recently, a major change in tannery equipment has been introduced in the form of a modified cement mixer with a ribbon flight to retain or remove stock at will. Additional features of the new processing equipment permits the recirculation of chemical solutions and their removal by pumping out to any designated location rather than to discharge them over a large floor area. Even as the completely tanned leather is ejected from the processor, the spent chrome liquor is easily confined to an area for recovery. Subsequent steps such as conveying to wringers are channeled over areas where drains convey all chrome containing liquors into one collection point.
There is no contamination of the spent chrome liquor because all other chemicals from washing, soaking, dehairing, bating, and washing are pumped out into the main sewers for handling in conventional waste treatment facilities without contamination by the chrome. By the same token, none of these chemicals can find their way into the segregated spent chrome liquor.
Many procedures have been tried whereby chromium compounds were processed for recycling. These have been amply reviewed in the Journal of the American Leather Chemists Association 67, 422-429 (1972) but are briefly described as follows:
1. Spent chrome liquor may be analyzed and brought up to original strength by the refortification. This requires exhaustive chemical analyses because the tanning liquors penetrate the stock at very low pH values and after penetration are "fixed" by neutralization to higher pH values. This results in a build-up of soluble salts so that a certain amount of the chrome liquor must be discarded each day to avoid this difficulty.
One of the ever present difficulties is to avoid loss of chromium compounds to the sewer as the amount of spent chrome liquor is prone to increase as it is constantly reacidified and reneutralized.
2. A modification of the above has been suggested which involves the use of spent chrome tan liquors along with the pickle step and proper adjustment of salts to retard swelling. To be sure, the spent chrome liquor can be exhausted in this way but only by extending the length of the pickling step to allow time for the chrome to be picked up by the hide material. Another problem that can occur is the premature precipitation of chromium as it penetrates the alkaline stock without sufficient advance pickling. This results in the formation of an impervious barrier which defeats the whole process. If one wishes to extend the tanning cycle by several hours then this procedure can be used. Of course, complete exhaustion of chrome liquors has been found to be unobtainable in a practical way.
3. Spent chrome liquor can also be used as the solvent for preparing sodium dichromate solutions prior to acidulation and reduction with glucose. However, here again, a constant check by chemical analyses must be rigidly maintained.
4. Precipitation of the chromium from spent chrome liquors as the hydroxide followed by filtration to remove soluble salts and proteins and final solution in sulfuric acid to the desired basicity has been found to be the only suitable procedure. This was used during World War II when a chromium shortage existed. Still required were constant chemical analyses and the ever present difficulty of filtering a flocculant thixotropic precipitate of variable composition. This procedure could be made to work but offered no economy under ordinary market conditions.
5. A modification of the above has also been tried. This includes the precipitation of chromium hydroxide from the spent liquors, allowing it to settle, decanting the clear supernatant salt solutions thereby reducing the total volume, redissolving the chromium hydroxide in sulfuric acid and fortifying with fresh chrome compounds to the desired composition and volume of solution. Here again, exhaustive analyses are necessary because flocculent chromium hydroxide rarely settles at the same rate and the precipitate as drawn off varies widely in concentration.
In every chrome recovery method, it is obvious that the spent chrome liquor must be screened and skimmed to remove most of the proteinaceous strings, flesh, and floatable fatty material.
It is, therefore, an object of this invention to provide a continuous and automatic method for the recovery and re-use of substantially all chromium from once-used spent chrome liquors. It is a further object of this invention to eliminate from chrome tannery effluents all chromium down to limits of one part per million. Still another object of the invention is to provide an automatically controlled continuous supply of recovered chromium tanning salts having a constant composition and requiring no batch analyses for determination of composition or concentration.
A further object of the invention is to provide a means for recovering approximately one-third of all the chrome tanning compounds used thereby effecting an economy of approximately 10 to 15 cents for every hide tanned.
Additional objects if not specifically set forth herein, will be readily apparent to those skilled in the art of chrome-tanning hides and skins.